Vehicles equipped with a friction clutch for engaging/disengaging the transmission of an engine drive force and a device for operating the friction clutch are well known. An example of a device used to operate a clutch is a clutch lever of a motorcycle.
Generally, as the size of a vehicle becomes larger, the capacity of the friction clutch becomes larger, and the force necessary to disengage the friction clutch also becomes larger. This, however, increases the load required to operate the clutch, which in turn impacts the operational burden on the rider. In order to reduce the load required to operate the clutch, and in turn the operational burden for the rider, a technique for attaching a so-called “power-assist device” to a friction clutch has been suggested, for example, in reference to Japanese Patent Application Publication 52-004955 and Japanese Patent 3381442.
A friction clutch with a power-assist device is disclosed in Japanese Patent 3381442. The friction clutch of Japanese Patent 3381442 includes a counter shaft (rotatable shaft), a clutch release hub (driven side rotating body) provided on the outer circumference of the rotatable shaft, a cylindrical clutch housing (drive side rotating body), which forms a contour of the clutch and has a bottom, a clutch disc (first plate) attached to the drive side rotating body, a clutch plate (second plate) which is attached to the driven side rotating body and is alternately disposed with the first plate, a clutch pressure disc (pressure plate) which presses the first plate and the second plate into contact with each other, an elastic member which biases the pressure plate in a direction in which the first plate and the second plate are pressed into contact with each other, a first clutch push piece (operating shaft) having a rod shape and a second clutch push piece of a substantially cylindrical shape which move the pressure plate in an axial direction of the rotatable shaft, a clutch operation system which moves the operating shaft in the axial direction of the rotatable shaft, a sub clutch including a sub clutch drive disc (pressing member) which rotates together with the drive side rotating body to move in the axial direction of the rotatable shaft, a sub clutch driven disc (torque transmission member) attached to one end of the operating shaft, and a friction member (friction plate) disposed between the pressing member and the torque transmission member in the axial direction of the rotatable shaft. The friction clutch includes a shift mechanism (power-assist device) which moves the pressure plate in a direction reverse to that of a predetermined direction by using torque transmitted by the sub clutch.
The pressure plate is pressed into contact with the outer circumference of the second clutch push piece. The pressing member is pressed into contact with an inner circumference of the second clutch push piece. Therefore, the pressure plate, the second clutch push piece and the pressing member can rotate together with the drive side rotating body. In addition, the pressure plate, the second clutch push piece and the pressing member can rotate together around the axis of rotation of the rotatable shaft. Therefore, the force necessary to disengage the clutch in the clutch operation system is solely the force required to move the pressing member in the axial direction of the rotatable shaft until the pressing member comes into pressure contact with the friction plate of the sub clutch. When the pressing member comes into pressure contact with the friction plate, the torque transmission member transmits torque to the power-assist device which moves the pressure plate in a direction for disengaging the clutch. As described above, the friction clutch can reduce the force necessary to disengage the clutch.
In the friction clutch disclosed in Japanese Patent 3381442, however, rotation of the drive side rotating body is transmitted to the pressing member in the sub clutch through the pressure plate and the second clutch push piece. In other words, the second clutch push piece is interposed between the pressure plate and the sub clutch. Therefore, an extra transmission path intervenes before rotation of the drive side rotating body is transmitted to the sub clutch. In addition, the second clutch push piece has a substantially cylindrical shape. In other words, the second clutch push piece is implemented as a member having a relatively large mass in the axial direction. Therefore, in constituting a transmission path between the pressure plate and the sub clutch, the outer shape requirements for the pressure plate and the pressing member become relatively complicated which complicates in turn the overall structure of the clutch.